Rotating polygon scanning mirrors are typically used in laser printers to provide a “raster” scan of the image of a laser light source across a moving photosensitive medium, such as a rotating drum. Such a system requires that the rotation of the photosensitive drum and the rotating polygon mirror be synchronized so that the beam of light (laser beam) sweeps or scans across the rotating drum in one direction as a facet of the polygon mirror rotates past the laser beam. The next facet of the rotating polygon mirror generates a similar scan or sweep which also traverses the rotating photosensitive drum but provides an image line that is spaced or displaced from the previous image line.
There have also been prior art efforts to use a less expensive flat mirror with a single reflective surface to provide a scanning beam. For example, a dual axis or single axis scanning mirror may be used to generate the beam sweep or scan instead of a rotating polygon mirror. The rotating photosensitive drum and the scanning mirror are synchronized as the drum rotates in a forward direction to produce a printed image line on the medium that is at right angles or orthogonal with the beam scan or sweep generated by the pivoting mirror.
However, with single axis mirrors the return sweep will traverse a trajectory on the moving photosensitive drum that is at an angle with the printed image line resulting from the previous or forward sweep. Consequently, use of a single axis resonant mirror, according to the prior art, required that the modulation of the reflected light beam be interrupted as the mirror completed the return sweep or cycle, and then turned on again as the beam starts scanning in the original direction. Using only one of the sweep directions of the mirror, of course, reduces the print speed. Therefore, to effectively use an inexpensive scanning mirror to provide bi-directional printing, the prior art typically required that the beam scan move in a direction perpendicular to the scan such that the sweep of the mirror in each direction generates images on a moving or rotating photosensitive drum that are always parallel. This continuous perpendicular adjustment is preferably accomplished by the use of a dual axis torsional mirror, but could be accomplished by using a pair of single axis torsional mirrors. It has been discovered, however, at today's high print speeds both forward and reverse sweeps of a single axis mirror may be used, and that no orthogonal adjustment is necessary.
Texas Instruments presently manufactures torsional dual axis and single axis resonant mirror MEMS devices fabricated out of a single piece of material (such as silicon, for example) typically having a thickness of about 100-115 microns. The dual axis layout consists of a mirror normally supported on a gimbal frame by two silicon torsional hinges, whereas for a single axis mirror the mirror is supported directly by a pair of torsional hinges. The reflective surface may be of any desired shape, although an elliptical shape having a long axis of about 4.0 millimeters and a short axis of about 1.5 millimeters is particularly useful. The elongated ellipse-shaped mirror is matched to the shape that the angle of the beam is received. The gimbal frame used by the dual axis mirror is attached to a support frame by another set of torsional hinges. These mirrors manufactured by Texas Instruments are particularly suitable for use as the scanning engine for high-speed laser printers and visual displays. These high-speed mirrors are also suitable for use as high-speed optical switches in communication systems. One example of a dual axis torsional hinged mirror is disclosed in U.S. Pat. No. 6,295,154 entitled “Optical Switching Apparatus” and was assigned to the same assignee on the present invention.
The present invention is particularly applicable to a mirror or reflective surface supported by torsional hinges and the discussion and embodiments are primarily with respect to mirrors. However, as suggested by the title and the above discussion, the invention is also applicable to “functional surfaces” other than mirrors that have a need for high-speed pivoting or oscillations. Therefore, functional surfaces other than mirrors may include light gratings as well as surfaces not concerned with light beams and the movement of light beams.
Therefore, it will be appreciated that, although many references and embodiment in the specification are with respect to mirrors, the claims are not to be so limited except for such specific limitations in the claims.
According to the prior art, torsional hinge devices were initially driven directly by magnetic coils interacting with small magnets mounted on the pivoting device at a location orthogonal to and away from the pivoting axis to oscillate the device or, in the case of a mirror functional surface, create the sweeping movement of the beam. In a similar manner, orthogonal movement of a beam sweep was also controlled by magnetic coils interacting with magnets mounted on the gimbals frame at a location orthogonal to the axis used to pivot the gimbals frame.
According to the earlier prior art, the magnetic coils controlling the functional surface or reflective surface portion of a mirror typically received an alternating positive and negative signal at a frequency suitable for oscillating the device at the desired rate. Little or no consideration was given to the resonant pivoting frequency of the device. Consequently, depending on the desired oscillating frequency or rate and the natural resonant frequency of the device about the pair of torsional hinges, significant energy could be required to pivot the device and especially to maintain the functional surface of the device in a state of oscillation. Furthermore, the magnets mounted on the functional surface portion added mass and limited the oscillating speed.
Later torsional devices, such as mirrors, were manufactured to have a specific resonant frequency substantially equivalent to the desired oscillation rate. Such resonant frequency devices were particularly useful when the functional surface of the devices was a mirror used as a scanning engine. Various inertially coupled drive techniques including the use of piezoelectric devices and electrostatic devices have been used to initiate and keep the functional surface or mirror oscillations at the resonant frequency.
It has now been discovered that the earlier inexpensive and dependable magnetic drive can also be used and set up in such a way to both maintain the pivoting device at its resonant frequency and to provide orthogonal movement. Unfortunately, the added mass of the magnets becomes more and more of a problem as the required frequency increases to meet the higher and higher speed demands. Further, the functional surface of a device can be of almost any shape, including square, round, elliptical, etc. However, an elongated elliptical shape has been found to be particularly suitable if the functional surface is a mirror. Unfortunately, these elongated elliptical-shaped mirrors introduce moment of inertia forces that result in excess flexing and bending of the mirror adjacent the hinges and tips of the mirror such that the mirror no longer meets the required “flatness” specifications for providing a satisfactory laser beam. The thickness of the mirror may be increased to maintain the necessary flatness, but the added weight and mass results in excess stress on the torsional hinges which can cause failures and/or reduced life.
Therefore, a scanning device having sufficient stiffness to maintain acceptable flatness at high oscillation speeds would be advantageous.